linux-kernel - Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

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Date:	Thu, 22 Aug 2013 07:56:26 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Cyrill Gorcunov" <gorcunov@...il.com>
Cc:	"Andy Lutomirski" <luto@...capital.net>,
	"David Vrabel" <david.vrabel@...rix.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	<Xen-devel@...ts.xen.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>,
	"Pavel Emelyanov" <xemul@...allels.com>,
	"Ingo Molnar" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts
 with existing use

>>> On 21.08.13 at 18:19, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, Aug 21, 2013 at 05:03:13PM +0100, Jan Beulich wrote:
>> > 
>> > Only to non-present ptes, as far as I know.
>> 
>> That's not really any guarantee. And the accessor functions also
>> don't check that they'd be used on non-present PTEs only.
> 
> Wait. This _PAGE_SWP_SOFT_DIRTY bit (which is in real PSE bit) assigned
> in only one place -- in try_to_unmap_one(). The PTE get non-present then
> and consists of swap entry format. I don't see any accessor to such entry
> without testing if it's swap entry or pte-none. What I'm missing?

Fact is that this

static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
{
	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
}

has no checking whatsoever that the PTE being modified is a
non-present one, not even in any of the debugging modes. It
would be a different thing if the above acted on a swp_entry_t.

The fact that there currently may be just a single call site (where
the caller guarantees the non-present state) is no guarantee that
in the future another one won't appear, and then result in very
hard to debug problems.

Jan

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